Conventionally, black pigments are used in cosmetics for makeup of specific parts such as eyeliners. Those black pigments used for cosmetics which are commonly known are carbon black, black iron oxide, and titanium black, etc. Of these black pigments, carbon black and titanium black are approved as coloring materials in some countries, but not in others, and their use as materials for cosmetics in the global market are largely limited.
On the other hand, black iron oxide is safe, and it is used globally as a black pigment for cosmetics, but its tinting strength is lower than that of carbon black, and a large amount must be added to the cosmetics to obtain sufficient blackness, so it does not provide much degree of freedom in the recipe. Attempts are made to improve the tinting strength of black iron oxide to solve this problem.
One way of improving the tinting strength of black iron oxide is to reduce the grain size. One method that has been presented for reducing particle size is a method of forming bubbles of inert gas in an aqueous solution containing ferrous ion to reduce the dissolved oxygen content in the aqueous solution, then, adding an alkaline material to generate ferrous hydroxide, heating the solution to a range of 60 to 100° C., and producing a magnetite microcrystal (triiron tetroxide) having a particle size of 0.01 to 0.1 μm (Patent Document 1). However, a decrease in the grain size increases the surface area. As a result, there will be a higher possibility of oxidation of the surface of black iron oxide when contacting air at a high temperature, which leads to generation of maghemite that gives a reddish color. Patent Document 1 teaches that the black iron oxide needs to be vacuum dried to obtain a fine black iron oxide, but when drying is performed industrially under air, the particles turn brown. Hence, it is not practical to use black iron oxide as a black pigment for cosmetics.
In a conventional production of black iron oxide, a problem of color change by heat is experienced in the drying step during production, and a problem in heat resistance, such as thermochromism, is experienced during use. A production method has been proposed for producing magnetite particle powder for black pigments by adding a ferrous salt solution to an alkali hydroxide solution, passing oxidative gas over it at 80 to 100° C. and adding an aluminum compound, then subjecting the obtained magnetite particles to filtration, washing, drying, burning and pulverizing (Patent Document 2). The magnetite particles obtained by addition of aluminum taught in Patent Document 2 has an octahedral shape, their average particle size is 0.1 to 0.2 μm, and the specific surface is 20 m2/g to 50 m2/g, but the tinting strength of the particles is not sufficient as black pigment. In Comparative Example 1, in which aluminum is not added, the specific surface was 11.3 m2/g, but the color turned reddish brown, and black pigment was not obtained.
A proposal has also been made of a black iron tetroxide having a particle size of 100 nm or lower and a specific surface of 40 m2/g or higher, which is obtained by dropping a water-soluble ferrous salt solution into an alkali hydroxide or an alkali carbonate solution, blowing air into the solution to obtain a fine goethite precipitate (iron(II) oxyhydroxide), and hydrogenising the goethite precipitate at 250 to 350° C. (Patent Document 3). The black iron tetroxide of Patent Document 3 takes the form of needle shaped particles that do not easily disperse, and are lacking in tinting strength.